Friction stir welding is one method for joining multiple members. In friction stir welding, a tool that has a protrusion at its tip is pressed onto the members while rotating; and the protrusion is inserted into the members. At the joining portion periphery where the protrusion is inserted, the main materials are softened by frictional heat; and plastic flow of the joining portion periphery is caused by the rotation of the tool. Thereby, the multiple members are formed as one body at the joining portion periphery.
A long fatigue life of the joining portion periphery is desirable for the members after the joining.